The present invention relates to an ultrasonic motor driven by the vibration of a piezoelectric element.
A typical conventional ultrasonic motor is illustrated in FIG. 1. The motor includes elastic vibrating member 2 which is vibrated by a piezoelectric resonator, and sliding member 4 mounted above member 2. Friction member 6 is adhered to member 4 and is in contact with member 2. The principle of driving of the ultrasonic motor will be described with reference to FIG. 2.
The vibration to member 2 is transferred of the resonator, as shown in FIG. 2. Member 2 generates elastic wave 8. Wave 8 propagates in the member 2 in the direction of arrow A. At material particle 12 on the surface of member 2, wave 8 propagates along an ellipse 13 having longitudinal amplitude C and transverse amplitude D. As wave 8 propagates in the direction of arrow A, it rotates counterlockwise (FIG. 2) along the ellipse. Wave 8 passes through vertex 10 for each wavelength, and member 6 is brought into tight contact with member 2 at vertex 10 of elastic wave 8. However, particle 12 is subjected to elliptical motion, and thus a vector acts on particle 12. The vector acting on particle 12 at vertex 10 is transferred by a frictional force created between members 2 and 6, thereby causing member 6 to move in a direction B opposite the propagation direction A of wave 8.
In linear or rotational ultrasonic motors of this type, a metal with a high Young's modulus is used for member 2 to improve propagation efficiency of the elastic wave. However, if a member 6 is made of a metal which is hard, elastic wave vibrations and a frictional force cause noise and heat. Hence, member 6 can be made of elastic rubber. However, if member 6 is made of elastic rubber, it absorbs the vibrations and the frictional force, and a sufficient driving force (i.e, torque) cannot be obtained.
Ultrasonic motors are described in Article Nos. 1-2-1, 1-2-2, and 1-2-3, Spring 1985, of the Institute of Acoustics of Japan. Each motor disclosed therein has an elastic resonator and a member of a material such as vinyl, polycarbonate, polypropylene, or duracon as a contact member with a frictional member in contact with the elastic resonance member. However, such materials are thermoplastic resins. In particular, if vinyl is in an ultrasonic motor and it is rotated at high speed for a long period of time, the physical properties of the vinyl are changed due to frictional heat, and its initial properties cannot be maintained. The physical properties of polypropylene are also degraded by heat. Polycarbonate cannot withstand repeated loading and has poor durability. Since duracon has a very low friction coefficient, it cannot obtain a sufficient frictional force from the elastic wave.